Method for shunt detection in sensors

ABSTRACT

For monitoring a sensor using differential voltage evaluation for detecting a short circuit to ground and/or to supply voltage U B , a first resistor and a second resistor R 1 , R 2  are assigned to the sensor, a sum voltage is determined from voltages U P , U M  at terminals of the sensor, the sum voltage is compared to supply voltage U B , a ratio a of determined sum voltage to supply voltage U B  is then calculated, and depending on the value for the ratio a, the occurrence of a shunt at the positive terminal and/or negative terminal of the sensor is detected.

BACKGROUND OF THE INVENTION

The present invention relates to a method for shunt detection in sensors.

Resistance-dependent semiconductor components (PTC elements) are used to record temperature values, which are evaluated via an analog input of a microprocessor. To increase accuracy, a difference evaluation is carried out, by way of which interfering ground effects can be eliminated. The temperature is required internally in the control devices to enable calculations to be carried out in the control device.

The method for performing a differential voltage evaluation is very widespread, in the automotive industry in particular, for use in the evaluation and determination of engine and transmission temperatures.

To prevent faulty temperature readings by temperature sensors, the operating method of temperature sensors is monitored. If temperatures are recorded in a faulty manner, erroneous calculations are carried out in control devices and incorrect characteristic curves are used. Generally speaking, a differential evaluation is carried out to detect a short circuit to ground, to U_(B) and an interruption. It has not been possible to detect shunts with methods used so far to monitor sensors. Shunts result in alterated differential voltage and, therefore, to a faulty temperature reading. As such, it is not sufficient to merely monitor temperature sensors for short circuit to ground, to U_(B), and detection of an interruption.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a method for shunt detection in sensors, which eliminates the disadvantages of the prior art.

In keeping with these objects and with others which will become apparent hereinafter, one feature of the present invention resides, briefly stated, in a method for monitoring a sensor using differential voltage evaluation for detecting a short circuit to ground and/or to supply voltage U_(B), comprising the steps of assigning a first resistor R₁ and a second resistor R₂ to the sensor; determining a sum voltage based on voltages U_(P), U_(M) at terminals of the sensor; comparing said sum voltage to the supply voltage U_(B); calculating a ratio a of the determined sum voltage to the supply voltage U_(B); and depending on a value for the ratio a, detecting an occurrence of a shunt at the sensor.

With the solution proposed according to the present invention, a shunt is detected at a sensor, e.g, a temperature sensor, by calculating the voltage values at the sensor. When the resistances at the positive and negative terminals of the sensor are the same, the sum of the voltage at the sensor is equal to the supply voltage. A ratio a of the sum voltage to supply voltage is therefore equal to 1, provided that R₁ and R₂ are the same.

This circumstance provides an opportunity for diagnostics to be improved, namely the diagnostics of shunt resistances in particular. The following tolerance intervals, for example, for the value of ratio a are practical choices for use in a diagnosis evaluation: As long as the value for a fluctuates between 0.95 and 1.05, a shunt is not present. A shunt error is detected as soon as the ratio a falls below 0.95 or exceeds 1.05, for example. Instead of the values of 0.95 and 1.05 for ratio a stated here as an example, values such as 0.9 and 1.1, respectively, can be selected, for instance; this depends on the accuracy requirement placed on a temperature sensor, for example. Depending on the requirements and specifications on the accuracy of the temperature reading, ratio a can be specified in a defined manner as a function of resistances R₁ and R₂.

The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic depiction of the sensor circuitry with resistors R₁ and R₂ assigned to said sensor,

FIG. 2 shows the course of ratio a when a shunt is present with resistance R_(M),

FIG. 3 shows ratio a when a shunt is present with resistance R_(P),

Table I is a table of values for ratios a that occur when a shunt is present with resistance R_(M) according to the depiction in FIG. 2, and

Table II shows the course of ratio a when a shunt is present with resistance R_(P), with a table of values.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The illustration shown in FIG. 1 is a schematic depiction of a sensor arrangement connected to a supply voltage source and two resistors R₁ and R₂.

A sensor 1, indicated in FIG. 1 as a variable sensor resistor R_(s), is connected to a supply voltage source, indicated as supply voltage U_(B), and is also connected to ground 4. A resistor R₂ is connected upstream of sensor 1, resistor R₂ being connected between ground 4 and a negative terminal 3 of the sensor. In addition, a first resistor R₁ is connected downstream of sensor 2, said first resistor R₁ being connected between a positive terminal 2 and supply voltage U_(P).

A shunt at positive terminal 2 is symbolized by shunt resistance R_(M), which is also connected to ground 4; a shunt at negative terminal 3 of sensor 1 is indicated by shunt resistance R_(M) at the negative terminal, and shunt resistance R_(M) is also connected to ground 4. Shunt resistances R_(M) and R_(P) are understood herein below to mean effective resistances, said shunt resistances being caused, e.g, by a conductive contamination at the sensor or leakage currents at a defective sensor cable. In the calculations shown below, R_(M) and R_(P) are considered to be real ohmic resistance. The values of resistances R_(M) and R_(P) are not known. U_(P), U_(M) and U_(B) are measured in the entire system, and the results are used to calculate ratio a. The value of ratio a is then compared with defined limits.

The following different cases can occur, based on FIG. 1:

Case 1 (No Shunt Resistance Present):

In this case, voltage U_(M) is present at negative terminal 3, said voltage being defined by the following relationship: $\begin{matrix} {U_{M} = {U_{B}\frac{R_{2}}{\left( {R_{S} + R_{1} + R_{2}} \right)}}} & \lbrack 1\rbrack \end{matrix}$ and, voltage U_(P) is present at positive terminal 2 of sensor 1, said voltage U_(P) being defined by the following relationship: $\begin{matrix} {U_{P} = {U_{B}\frac{R_{2} + R_{S}}{\left( {R_{S} + R_{1} + R_{2}} \right)}}} & \lbrack 2\rbrack \end{matrix}$

Ratio a of the sum voltage to supply voltage is defined by: $\begin{matrix} {a = \frac{U_{p} + U_{M}}{U_{B}}} & \lbrack 3\rbrack \end{matrix}$

When [1] and [2] are substituted in [3], ratio a is defined by: $a = {\frac{\left( {R_{2} + R_{S}} \right) + R_{2}}{R_{S} + R_{1} + R_{2}} = \frac{{2R_{2}} + R_{S}}{R_{S} + R_{1} + R_{2}}}$

When R₁=R₂=R, ratio a is always =1.

Case 2 (with Shunt Resistance R_(M) at Negative Terminal 3):

For simplicity, it is assumed that: $R_{X} = {{R_{2}\bullet\quad R_{M}} = \frac{R_{2}R_{M}}{R_{2} + R_{M}}}$

Ratio a is obtained, based on the equation above, as follows: $a = \frac{{2R_{X}} + R_{S}}{R_{S} + R_{1} + R_{X}}$

Therefore: $\begin{matrix} {a = \frac{{2R_{2}R_{M}} + {R_{S}\left( {R_{2} + R_{M}} \right)}}{{R_{1}\left( {R_{2} + R_{M}} \right)} + {R_{S}\left( {R_{2} + R_{M}} \right)} + {R_{2}R_{M}}}} \\ {= \frac{{R_{M}\left( {{2R_{2}} + R_{S}} \right)} + {R_{S}R_{2}}}{{R_{M}\left( {R_{1} + R_{2} + R_{S}} \right)} + {R_{2}\left( {R_{1} + R_{S}} \right)}}} \end{matrix}$

If R₁=R₂ =R, ratio a is equal to: $a = \frac{{R_{M}\left( {{2R} + R_{S}} \right)} + {RR}_{S}}{{R_{M}\left( {{2R} + R_{S}} \right)} + {R\left( {R + R_{S}} \right)}}$

For the case in which R_(M)=∝, ratio a is equal to 1.

Case 3 (with Shunt Resistance R_(P) at Positive Terminal 2)

In this case, in which shunt resistance R_(P) is present at positive terminal 2 of sensor 1 according to the illustration in FIG. 1, the following applies: $U_{P} = {U_{B}\frac{\frac{R_{P}\left( {R_{2} + R_{S}} \right)}{R_{P} + R_{2} + R_{S}}}{\frac{R_{P}\left( {R_{2} + R_{S}} \right)}{R_{P} + R_{2} + R_{S}} + R}}$

As a result, U_(P), i.e, voltage U_(P) present at positive terminal 2, is defined as: $\begin{matrix} {U_{P} = {U_{B}\frac{R_{P}\left( {R_{2} + R_{S}} \right)}{{R_{P}\left( {R_{2} + R_{S}} \right)} + {R_{1}\left( {R_{P} + R_{2} + R_{S}} \right)}}}} & \lbrack 1\rbrack \end{matrix}$

Voltage U_(M) present at negative terminal 3 is defined as: $\begin{matrix} {U_{M} = {U_{P}\frac{R_{2}}{R_{2} + R_{S}}}} & \lbrack 2\rbrack \end{matrix}$

Ratio a of sum voltage to supply voltage is defined as follows: $a = {\frac{U_{P} + {U_{P}\frac{R_{2}}{R_{2} + R_{S}}}}{U_{B}} = \frac{U_{P}\left( \left( {1 + \frac{R_{2}}{R_{2} + R_{S}}} \right) \right)}{U_{B}}}$

Based on equation [1], ratio a is defined as follows: $a = {\frac{R_{P}\left( {{2R_{2}} + R_{S}} \right)}{{R_{1}\left( {R_{P} + R_{2} + R_{S}} \right)} + {R_{P}\left( {R_{2} + R_{S}} \right)}} =}$

For the case in which first resistance R₁ is equal to second resistance R₂, the following applies: R₁=R₂=R:

Ratio a of sum voltage to supply voltage is therefore defined as follows: $a = \frac{R_{P}\left( {{2R} + R_{S}} \right)}{{R_{P}\left( {{2R} + R_{S}} \right)} + {R\left( {R + R_{S}} \right)}}$

For the case in which R_(P) has the value ∝, ratio a of sum voltage to supply voltage is equal to 1.

If the shunt resistances, i.e, R_(M) and R_(P), are connected to supply voltage U_(P), the relationships become reversed, so that values >1 result for ratio a. The values indicated below are practical choices for use in performing a diagnosis evaluation; these values are selected as examples only, however:

For the case in which the value of ratio a is ≦1.05 but ≧0.95, i.e, 0.95≦a≦1.05, the diagnosis is “no shunt”.

If the value of ratio a is less than 0.95 or greater than 1.05, the diagnosis is that a shunt error has occurred, i.e, a<0.95 or a>1.05.

In the illustration shown in FIG. 2, ratio a with a shunt having shunt resistance R_(M) is plotted against various values for shunt resistance R_(M). The various curve traces are plotted for a variable sensor resistance of 50, 100, 200, 400, 600, 800, 1000, 1200, 1400, 1600, 1800 and 2 kOhm. The values for first resistance R₁ and second resistance R₂ are both 1 kOhm.

The values that occur in this case are listed in Table I. The individual values for sensor resistance R_(S) of 50 Ohm to 2 kOhm are listed in the first row of the table of values. The values for shunt resistance R_(M) of 0 to 50000 are listed in the column to the far left.

In the illustration in FIG. 3, the course of ratio a with a shunt having shunt resistance R_(P) at positive terminal 2 of sensor 1 is shown. In the case depicted in FIG. 3, sensor resistance R_(S) has values 50 Ohm, 100 Ohm, 200 Ohm, 400 Ohm, 600 Ohm, 800 Ohm, 1000 Ohm, 1200 Ohm, 1400 Ohm, 1600 Ohm, 1800 Ohm, and 2 kOhm, and ratio a is between 0.8 and 1. The values for first resistance R₁ and second resistance R₂ are also each equal to 1 kOhm in the curve traces shown in FIG. 3.

The values of ratio a that occurs with a shunt having resistance R_(P) at positive terminal 2 that belong to FIG. 3 are shown in Table II. As with Table I, the changing shunt resistance R_(P) from 0 to 50000 Ohm is listed in the column to the far left, and variable sensor resistances R_(S) of 50, 100, 200, 400, 600, 800 Ohm, 1 kOhm, 1.2 kOhm, 1.4 kOhm, 1.6 kOhm, 1.8 kOhm and 2 kOhm are listed in the top row in Table II. The course of ratio a with a shunt having shunt resistance R_(P) at positive terminal 2 is based on the values in Table II. It becomes clear that, similar to the depiction according to FIGS. 2, 3, the curve traces that occur for ratio a with a shunt resistance R_(M) and with a shunt resistance R_(P)—which are selected to be between 0 and 50 kOhm—result in a large number of curve traces. All of them have a steep slope that transitions into an asymptotic course approaching 1.

Values for ratio a for which a sensor evaluation should be carried out are between 0.95 and 1.05, as mentioned above. In other words, they lie in a range in which shunt resistance R_(M) and shunt resistance R_(P) are ≧10 kOhm.

The exact limits of a at which a sensor evaluation must still take place depend on the accuracy requirements placed on the overall system. If values for a are outside this defined range, the influence of the shunt resistance on the overall system is so great that it no longer makes sense to perform the sensor evaluation and, if necessary, a sensor replacement value can be utilized instead. In this case, the sensor diagnosis detects a shunt error. TABLE I Variable Sensor Resistance R_(s) Shunt 50 100 200 400 600 800 1000 50000 0.99034283 0.99057493 0.99100719 0.99176277 0.99240122 0.99294781 0.99342105 45000 0.98928189 0.98963975 0.99001996 0.99085923 0.9915683 0.99217527 0.99270073 40000 0.98796906 0.98824912 0.98878924 0.98973306 0.9905303 0.99121265 0.99180328 35000 0.98626374 0.98659517 0.98721228 0.9882904 0.98920086 0.98997996 0.99065421 30000 0.98401279 0.98439938 0.98511905 0.98637602 0.98743719 0.98834499 0.98913043 25000 0.98087954 0.98134328 0.98220641 0.98371336 0.98498498 0.98607242 0.98701299 20000 0.97621879 0.97679814 0.97787611 0.9795709 0.98134328 0.98269896 0.98387097 15000 0.96855346 0.96932515 0.97076023 0.97326203 0.97536946 0.97716895 0.9787234 10000 0.96359629 0.95475113 0.95689655 0.96062992 0.96376812 0.96664495 0.96875 8000 0.94269341 0.94413408 0.94680851 0.95145631 0.95535714 0.95867769 0.96153846 6000 0.92509363 0.9270073 0.93055556 0.93670886 0.94186047 0.94623656 0.95 5000 0.91150442 0.9137931 0.91803279 0.92537313 0.93150685 0.93670886 0.94117647 4000 0.89189189 0.89473684 0.9 0.90909091 0.91666667 0.92307692 0.92857143 3000 0.86111111 0.86486486 0.87179487 0.88372093 0.89361702 0.90196078 0.90909091 2000 0.80582524 0.81132075 0.82142857 0.83870968 0.85294118 0.86486486 0.875 1000 0.67741935 0.6875 0.70588235 0.73684211 0.76190476 0.7826087 0.8 0 0.04761905 0.09090909 0.1666667 0.28571429 0.375 0.44444444 0.5 Shunt 1200 1400 1600 1800 2000 50000 0.99318477 0.99419954 0.99452355 0.99481328 0.99507389 45000 0.99316005 0.99356499 0.99392467 0.99424626 0.99453552 40000 0.99231951 0.99277457 0.99317872 0.99354005 0.99386503 35000 0.99124343 0.99176277 0.99222395 0.99263623 0.99300699 30000 0.9898167 0.99042146 0.99095841 0.99143836 0.99186992 25000 0.98783455 0.98855835 0.98920066 0.98777505 0.99029126 20000 0.9848926 0.98579545 0.98659517 0.98730964 0.98795181 15000 0.98007968 0.98127341 0.98233216 0.98327759 0.98412698 10000 0.97076023 0.97252747 0.97409326 0.9754902 0.97674419 8000 0.96402878 0.96621622 0.96815287 0.96987952 0.97142857 6000 0.95327103 0.95614035 0.95867769 0.9609375 0.96296296 5000 0.94505495 0.94845361 0.95145631 0.95412844 0.95652174 4000 0.93333333 0.9375 0.94117647 0.94444444 0.94736842 3000 0.91525424 0.92063492 0.92537313 0.92957746 0.93333333 2000 0.88372093 0.89130435 0.89795918 0.90384615 0.90909091 1000 0.81481481 0.82758621 0.83870968 0.84848485 0.85714286 0 0.54545455 0.58333333 0.61538462 0.64285714 0.66666667

TABLE II Variable Sensor Resistance R_(s) Shunt resistance 50 100 200 400 600 800 1000 50000 0.98985997 0.98963242 0.98920863 0.98846787 0.98784195 0.98730606 0.98684211 45000 0.98874598 0.98849372 0.98802395 0.98720293 0.98650927 0.9591549 0.98540146 40000 0.98735701 0.98707403 0.98654709 0.98562628 0.98484848 0.98418278 0.98360656 35000 0.98557692 0.98625469 0.98465473 0.98360656 0.98272138 0.98196393 0.98130841 30000 0.98321343 0.98283931 0.98214286 0.98092643 0.9798995 0.97902098 0.97826087 25000 0.97992352 0.97947761 0.97864769 0.9771987 0.97597598 0.97493036 0.974025797 20000 0.97502973 0.97447796 0.97345133 0.97165992 0.97014925 0.96885813 0.96774194 15000 0.96698113 0.96625767 0.96491228 0.96256684 0.96059113 0.95890411 0.95744681 10000 0.9512761 0.95022624 0.94827586 0.94488189 0.94202899 0.93959732 0.9375 8000 0.93982808 0.93854749 0.93617021 0.93203883 0.92857143 0.92561983 0.92307692 6000 0.92134831 0.91970803 0.91666667 0.91139241 0.90697674 0.90322581 0.9 5000 0.90707965 0.90517241 0.90163934 0.89562239 0.89041096 0.88607595 0.88235294 4000 0.88648649 0.88421053 0.88 0.87282727 0.86666667 0.86153846 0.85714286 3000 0.85416667 0.85135135 0.84615385 0.8372093 0.82978723 0.82352941 0.81818182 2000 0.791165 0.79245283 0.78571429 0.77419355 0.76470588 0.75675676 0.75 1000 0.66129032 0.65625 0.64705882 0.63157895 0.61904762 0.60869565 0.6 0 0 0 0 0 0 0 0 Shunt resistance 1200 1400 1600 1800 2000 50000 0.986436498 0.98607889 0.98576123 0.98547718 0.98522167 45000 0.98495212 0.98455598 0.98420413 0.98388953 0.98360656 40000 0.983102919 0.98265896 0.98226467 0.98191244 0.9815909 35000 0.980735552 0.98023046 0.97978227 0.97938144 0.97902098 30000 0.977596741 0.97701149 0.97649186 0.9760274 0.97560976 25000 0.97323601 0.9725490S 0.97192225 0.97137014 0.97087379 20000 0.966767372 0.96590909 0.96514745 0.96446701 0.96385542 15000 0.956175299 0.95505618 0.9540636 0.95317726 0.96238095 10000 0.935672515 0.93406593 0.93264249 0.93137255 0.93023256 8000 0.920863309 0.91891892 0.91719745 0.91566265 0.91428571 6000 0.897196262 0.89473684 0.89256198 0.890625 0.88888889 5000 0.879120879 0.87628866 0.87378641 0.87156963 0.86956622 4000 0.853333333 0.85 0.84705882 0.84444444 0.84210526 3000 0.813559322 0.80952381 0.80597015 0.8028169 0.8 2000 0.744186047 0.73913043 0.73469388 0.73076923 0.72727273 1000 0.592592593 0.5862069 0.58064516 0.57575758 0.57142867 0 0 0 0 0 0 

1. A method for monitoring a sensor using differential voltage evaluation for detecting a short circuit to ground and/or to supply voltage U_(B), comprising the steps of assigning a first resistor R₁ and a second resistor R₂ to the sensor; determining a sum voltage based on voltages U_(P), U_(M) at terminals of the sensor; comparing said sum voltage to the supply voltage U_(B); calculating a ratio a of the determined sum voltage to the supply voltage U_(B); and depending on a value for the ratio a, detecting an occurrence of a shunt at the sensor.
 2. A method as defined recited in claim 1; and further comprising selecting the ratio $a = \frac{U_{P} + U_{M}}{U_{B}}$ equals 1 when values for the first resistance R₁ and the second resistance R₂, are the same.
 3. A method as defined in claim 1; and further comprising determining an occurrence of a shunt at a negative terminal to ground at the sensor by a ratio a based on a relationship: $a = \frac{{R_{M}\left( {{2R} + R_{S}} \right)} + {RR}_{S}}{{R_{M}\left( {{2R} + R_{S}} \right)} + {R\left( {R + R_{S}} \right)}}$ with R₁=R₂=R.
 4. A method as recited in claim 1; and further comprising determining an occurrence of a shunt at a positive terminal to ground at the sensor by a ratio a based on a relationship: $a = \frac{R_{P}\left( {{2R} + R_{S}} \right)}{{R_{P}\left( {{2R} + R_{S}} \right)} + {R\left( {R + R_{S}} \right)}}$ with R₁=R₂=R.
 5. A method as defined in claim 3; and further comprising determining an occurrence of a shunt at the negative terminal to U_(B) at the sensor by 1/a.
 6. A method as recited in claim 4; and further comprising determining an occurrence of a shunt at the positive terminal to U_(B) at the sensor by 1/a.
 7. A method as defined in claim 1; and further comprising indicating by values of 0.95≦a≦1.05 that a shunt is not present.
 8. A method as defined in claim 1; and further comprising indicating by values of a<0.95 or a<1.05 that a shunt is present at a positive terminal or a negative terminal of the sensor. 